Detection of device removal from a surface of a multi-coil wireless charging device

ABSTRACT

Systems, methods and apparatus for wireless charging are disclosed. A charging device has a plurality of charging cells provided on a charging surface, a charging circuit and a controller. The controller may be configured to cause the charging circuit to provide a charging current to a resonant circuit when a receiving device is placed on the charging surface, detect a change or rate of change in voltage or current level associated with the resonant circuit, provide a measurement slot by terminating the charging current for a period of time, and determine that the receiving device has been removed from the charging surface by performing a passive or digital ping procedure during the measurement slot.

PRIORITY CLAIM

This application claims priority to and the benefit of provisionalpatent application No. 62/877,831 filed in the United States PatentOffice on Jul. 23, 2019, of provisional patent application No.63/019,241 filed in the United States Patent Office on May 1, 2020, ofprovisional patent application No. 63/019,245 filed in the United StatesPatent Office on May 1, 2020, and of provisional patent application No.63/019,248 filed in the United States Patent Office on May 1, 2020, theentire content of which applications are incorporated herein byreference as if fully set forth below in their entirety and for allapplicable purposes.

TECHNICAL FIELD

The present invention relates generally to wireless charging ofbatteries, including batteries in mobile computing devices, and moreparticularly to detection of device removal during a charging operation.

BACKGROUND

Wireless charging systems have been deployed to enable certain types ofdevices to charge internal batteries without the use of a physicalcharging connection. Devices that can take advantage of wirelesscharging include mobile processing and/or communication devices.Standards, such as the Qi standard defined by the Wireless PowerConsortium enable devices manufactured by a first supplier to bewirelessly charged using a charger manufactured by a second supplier.Standards for wireless charging are optimized for relatively simpleconfigurations of devices and tend to provide basic chargingcapabilities.

Improvements in wireless charging capabilities are required to supportcontinually increasing complexity of mobile devices and changing formfactors. For example, there is a need for a faster, lower powerdetection techniques that enable a charging device to detect and locatechargeable devices on a surface of a charging device, and to detectremoval or relocation of a chargeable device during a wireless chargingoperation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a charging cell that may be provided ona charging surface provided by a wireless charging device in accordancewith certain aspects disclosed herein.

FIG. 2 illustrates an example of an arrangement of charging cellsprovided on a single layer of a segment of a charging surface providedby a wireless charging device in accordance with certain aspectsdisclosed herein.

FIG. 3 illustrates an example of an arrangement of charging cells whenmultiple layers of charging cells are overlaid within a segment of acharging surface provided by a wireless charging device in accordancewith certain aspects disclosed herein.

FIG. 4 illustrates the arrangement of power transfer areas provided by acharging surface of a charging device that employs multiple layers ofcharging cells configured in accordance with certain aspects disclosedherein.

FIG. 5 illustrates a wireless transmitter that may be provided in acharger base station in accordance with certain aspects disclosedherein.

FIG. 6 illustrates a first example of a response to a passive ping inaccordance with certain aspects disclosed herein.

FIG. 7 illustrates a second example of a response to a passive ping inaccordance with certain aspects disclosed herein.

FIG. 8 illustrates examples of observed differences in responses to apassive ping in accordance with certain aspects disclosed herein.

FIG. 9 illustrates a first topology that supports matrix multiplexingswitching for use in a wireless charger adapted in accordance withcertain aspects disclosed herein.

FIG. 10 illustrates a second topology that supports direct current drivein a wireless charger adapted in accordance with certain aspectsdisclosed herein.

FIG. 11 illustrates a multi-coil wireless charging system configured toreliably detect removal of a receiving device in accordance with certainaspects of this disclosure.

FIG. 12 is a graphical representation of certain aspects of a deviceremoval event that may be monitored in accordance with certain aspectsdisclosed herein.

FIG. 13 illustrates a filtered threshold detection circuit that employslow-pass filtering to accommodate variability in charging current ortank voltage in accordance with certain aspects disclosed herein.

FIG. 14 illustrates a Q-factor comparison circuit used for detection ofremoval of a receiving device in accordance with certain aspectsdisclosed herein.

FIG. 15 illustrates the use of look-up tables for detecting a deviceremoval event in accordance with certain aspects disclosed herein.

FIG. 16 illustrates an example of a procedure that uses look-up tableswhen detecting a device removal event in accordance with certain aspectsdisclosed herein.

FIG. 17 illustrates the use of measured quiescent or idle transfer powerdraw for detecting a device removal event in accordance with certainaspects disclosed herein.

FIG. 18 illustrates a first example of a procedure for device removaldetection based on measured quiescent power draw according to certainaspects disclosed herein.

FIG. 19 illustrates a second example of a procedure for device removaldetection based on measured quiescent power draw according to certainaspects disclosed herein.

FIG. 20 illustrates a first example of the use of a measurement slot toperform a ping procedure according to certain aspects disclosed herein.

FIG. 21 illustrates a second example of the use of a measurement slot toperform a ping procedure according to certain aspects disclosed herein.

FIG. 22 illustrates a first example of the use of sensors to detectremoval of a receiving device during power transfer in accordance withcertain aspects disclosed herein.

FIG. 23 illustrates a second example of the use of sensors to detectremoval of a receiving device during power transfer in accordance withcertain aspects disclosed herein.

FIG. 24 illustrates a third example of the use of sensors to detectremoval of a receiving device during power transfer in accordance withcertain aspects disclosed herein.

FIG. 25 illustrates a fourth example of the use of sensors to detectremoval of a receiving device during power transfer in accordance withcertain aspects disclosed herein.

FIG. 26 illustrates a fifth example of the use of sensors to detectremoval of a receiving device during power transfer in accordance withcertain aspects disclosed herein.

FIG. 27 illustrates one example of an apparatus employing a processingcircuit that may be adapted according to certain aspects disclosedherein.

FIG. 28 illustrates a method for operating a charging device inaccordance with certain aspects of this disclosure.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various configurations and isnot intended to represent the only configurations in which the conceptsdescribed herein may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof various concepts. However, it will be apparent to those skilled inthe art that these concepts may be practiced without these specificdetails. In some instances, well known structures and components areshown in block diagram form in order to avoid obscuring such concepts.

Several aspects of wireless charging systems will now be presented withreference to various apparatus and methods. These apparatus and methodswill be described in the following detailed description and illustratedin the accompanying drawing by various blocks, modules, components,circuits, steps, processes, algorithms, etc. (collectively referred toas “elements”). These elements may be implemented using electronichardware, computer software, or any combination thereof. Whether suchelements are implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem.

By way of example, an element, or any portion of an element, or anycombination of elements may be implemented with a “processing system”that includes one or more processors. Examples of processors includemicroprocessors, microcontrollers, digital signal processors (DSPs),field programmable gate arrays (FPGAs), programmable logic devices(PLDs), state machines, gated logic, discrete hardware circuits, andother suitable hardware configured to perform the various functionalitydescribed throughout this disclosure. One or more processors in theprocessing system may execute software. Software shall be construedbroadly to mean instructions, instruction sets, code, code segments,program code, programs, subprograms, software modules, applications,software applications, software packages, routines, subroutines,objects, executables, threads of execution, procedures, functions, etc.,whether referred to as software, firmware, middleware, microcode,hardware description language, or otherwise. The software may reside ona processor-readable storage medium. A processor-readable storagemedium, which may also be referred to herein as a computer-readablemedium may include, by way of example, a magnetic storage device (e.g.,hard disk, floppy disk, magnetic strip), an optical disk (e.g., compactdisk (CD), digital versatile disk (DVD)), a smart card, a flash memorydevice (e.g., card, stick, key drive), Near Field Communications (NFC)token, random access memory (RAM), read only memory (ROM), programmableROM (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM),a register, a removable disk, a carrier wave, a transmission line, andany other suitable medium for storing or transmitting software. Thecomputer-readable medium may be resident in the processing system,external to the processing system, or distributed across multipleentities including the processing system. Computer-readable medium maybe embodied in a computer-program product. By way of example, acomputer-program product may include a computer-readable medium inpackaging materials. Those skilled in the art will recognize how best toimplement the described functionality presented throughout thisdisclosure depending on the particular application and the overalldesign constraints imposed on the overall system.

Overview

Certain aspects of the present disclosure relate to systems, apparatusand methods applicable to wireless charging devices and techniques.Charging cells may be configured with one or more inductive coils toprovide a charging surface in a charging device where the chargingsurface enables the charging device to charge one or more chargeabledevices wirelessly. The location of a device to be charged may bedetected through sensing techniques that associate location of thedevice to changes in a physical characteristic centered at a knownlocation on the charging surface. Sensing of location may be implementedusing capacitive, resistive, inductive, touch, pressure, load, strain,and/or another appropriate type of sensing.

In one aspect of the disclosure, an apparatus has a battery chargingpower source, a plurality of charging cells configured in a matrix, afirst plurality of switches in which each switch is configured to couplea row of coils in the matrix to a first terminal of the battery chargingpower source, and a second plurality of switches in which each switch isconfigured to couple a column of coils in the matrix to a secondterminal of the battery charging power source. Each charging cell in theplurality of charging cells may include one or more coils surrounding apower transfer area. The plurality of charging cells may be arrangedadjacent to the charging surface of the charging device without overlapof power transfer areas of the charging cells in the plurality ofcharging cells.

In some instances, the apparatus may also be referred to as a chargingsurface. Power can be wirelessly transferred to a receiving devicelocated anywhere on a surface of the apparatus. The devices can have anarbitrarily defined size and/or shape and may be placed without regardto any discrete placement locations enabled for charging. Multipledevices can be simultaneously charged on a single charging surface. Theapparatus can track motion of one or more devices across the chargingsurface.

Charging Cells

According to certain aspects disclosed herein, a charging surface may beprovided using charging cells in a charging device, where the chargingcells are deployed adjacent to the charging surface. In one example thecharging cells are deployed in one or more layers of the chargingsurface in accordance with a honeycomb packaging configuration. Acharging cell may be implemented using one or more coils that can eachinduce a magnetic field along an axis that is substantially orthogonalto the charging surface adjacent to the coil. In this description, acharging cell may refer to an element having one or more coils whereeach coil is configured to produce an electromagnetic field that isadditive with respect to the fields produced by other coils in thecharging cell and directed along or proximate to a common axis.

In some implementations, a charging cell includes coils that are stackedalong a common axis and/or that overlap such that they contribute to aninduced magnetic field substantially orthogonal to the charging surface.In some implementations, a charging cell includes coils that arearranged within a defined portion of the charging surface and thatcontribute to an induced magnetic field within the substantiallyorthogonal portion of the charging surface associated with the chargingcell. In some implementations, charging cells may be configurable byproviding an activating current to coils that are included in adynamically-defined charging cell. For example, a charging device mayinclude multiple stacks of coils deployed across the charging surface,and the charging device may detect the location of a device to becharged and may select some combination of stacks of coils to provide acharging cell adjacent to the device to be charged. In some instances, acharging cell may include, or be characterized as a single coil.However, it should be appreciated that a charging cell may includemultiple stacked coils and/or multiple adjacent coils or stacks ofcoils. The coils may be referred to herein as charging coils, wirelesscharging coils, transmitter coils, transmitting coils, powertransmitting coils, power transmitter coils, or the like.

FIG. 1 illustrates an example of a charging cell 100 that may bedeployed and/or configured to provide a charging surface of a chargingdevice. As described herein, the charging surface may include an arrayof charging cells 100 provided on one or more substrates 106. A circuitcomprising one or more integrated circuits (ICs) and/or discreteelectronic components may be provided on one or more of the substrates106. The circuit may include drivers and switches used to controlcurrents provided to coils used to transmit power to a receiving device.The circuit may be configured as a processing circuit that includes oneor more processors and/or one or more controllers that can be configuredto perform certain functions disclosed herein. In some instances, someor all of the processing circuit may be provided external to thecharging device. In some instances, a power supply may be coupled to thecharging device.

The charging cell 100 may be provided in close proximity to an outersurface area of the charging device, upon which one or more devices canbe placed for charging. The charging device may include multipleinstances of the charging cell 100. In one example, the charging cell100 has a substantially hexagonal shape that encloses one or more coils102, which may be constructed using conductors, wires or circuit boardtraces that can receive a current sufficient to produce anelectromagnetic field in a power transfer area 104. In variousimplementations, some coils 102 may have a shape that is substantiallypolygonal, including the hexagonal charging cell 100 illustrated inFIG. 1. Other implementations provide coils 102 that have other shapes.The shape of the coils 102 may be determined at least in part by thecapabilities or limitations of fabrication technology, and/or tooptimize layout of the charging cells on a substrate 106 such as aprinted circuit board substrate. Each coil 102 may be implemented usingwires, printed circuit board traces and/or other connectors in a spiralconfiguration. Each charging cell 100 may span two or more layersseparated by an insulator or substrate 106 such that coils 102 indifferent layers are centered around a common axis 108.

FIG. 2 illustrates an example of an arrangement 200 of charging cells202 provided on a single layer of a segment of a charging surface of acharging device that may be adapted in accordance with certain aspectsdisclosed herein. The charging cells 202 are arranged according to ahoneycomb packaging configuration. In this example, the charging cells202 are arranged end-to-end without overlap. This arrangement can beprovided without through-hole or wire interconnects. Other arrangementsare possible, including arrangements in which some portion of thecharging cells 202 overlap. For example, wires of two or more coils maybe interleaved to some extent.

FIG. 3 illustrates an example of an arrangement of charging cells fromtwo perspectives 300, 310 when multiple layers are overlaid within asegment of a charging surface that may be adapted in accordance withcertain aspects disclosed herein. Layers of charging cells 302, 304,306, 308 are provided within a segment of a charging surface. Thecharging cells within each layer of charging cells 302, 304, 306, 308are arranged according to a honeycomb packaging configuration. In oneexample, the layers of charging cells 302, 304, 306, 308 may be formedon a printed circuit board that has four or more layers. The arrangementof charging cells 100 can be selected to provide complete coverage of adesignated charging area that is adjacent to the illustrated segment.

FIG. 4 illustrates the arrangement of power transfer areas provided in acharging surface 400 that employs multiple layers of charging cellsconfigured in accordance with certain aspects disclosed herein. Theillustrated charging surface is constructed from four layers of chargingcells 402, 404, 406, 408. In FIG. 4, each power transfer area providedby a charging cell in the first layer of charging cells 402 is marked“L1”, each power transfer area provided by a charging cell in the secondlayer of charging cells 404 is marked “L2”, each power transfer areaprovided by a charging cell in the third layer of charging cells 406 ismarked “L3”, and each power transfer area provided by a charging cell inthe fourth layer of charging cells 408 is marked “L4”.

Wireless Transmitter

FIG. 5 illustrates a wireless transmitter 500 that may be provided in acharger base station. A controller 502 may receive a feedback signalfiltered or otherwise processed by a conditioning circuit 508. Thecontroller may control the operation of a driver circuit 504 thatprovides an alternating current to a resonant circuit 506 that includesa capacitor 512 and inductor 514. The resonant circuit 506 may also bereferred to herein as a tank circuit, LC tank circuit, or LC tank, andthe voltage 516 measured at an LC node 510 of the resonant circuit 506may be referred to as the tank voltage.

The wireless transmitter 500 may be used by a charging device todetermine if a compatible device has been placed on a charging surface.For example, the charging device may determine that a compatible devicehas been placed on the charging surface by sending an intermittent testsignal (active ping) through the wireless transmitter 500, where theresonant circuit 506 may detect or receive encoded signals when acompatible device responds to the test signal. The charging device maybe configured to activate one or more coils in at least one chargingcell after receiving a response signal defined by standard, convention,manufacturer or application. In some examples, the compatible device canrespond to a ping by communicating received signal strength such thatthe charging device can find an optimal charging cell to be used forcharging the compatible device.

Passive ping techniques may use the voltage and/or current measured orobserved at the LC node 510 to identify the presence of a receiving coilin proximity to the charging pad of a device adapted in accordance withcertain aspects disclosed herein. In many conventional wireless chargertransmitters, circuits are provided to measure voltage at the LC node510 or to measure the current in the LC network. These voltages andcurrents may be monitored for power regulation purposes or to supportcommunication between devices. In the example illustrated in FIG. 5,voltage at the LC node 510 is monitored, although it is contemplatedthat current may additionally or alternatively be monitored to supportpassive ping in which a short pulse is provided to the resonant circuit506. A response of the resonant circuit 506 to a passive ping (initialvoltage V₀) may be represented by the voltage (V_(LC)) at the LC node510, such that:

$\begin{matrix}{V_{LC} = {V_{0}e^{{- {(\frac{\omega}{2Q})}}t}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

According to certain aspects disclosed herein, coils in one or morecharging cells may be selectively activated to provide an optimalelectromagnetic field for charging a compatible device. In someinstances, coils may be assigned to charging cells, and some chargingcells may overlap other charging cells. In the latter instances, theoptimal charging configuration may be selected at the charging celllevel. In other instances, charging cells may be defined based onplacement of a device to be charged on a surface of the charging device.In these other instances, the combination of coils activated for eachcharging event can vary. In some implementations, a charging device mayinclude a driver circuit that can select one or more cells and/or one ormore predefined charging cells for activation during a charging event.

FIG. 6 illustrates a first example in which a response 600 to a passiveping decays according to Equation 3. After the excitation pulse at timet=0, the voltage and/or current is seen to oscillate at the resonantfrequency defined by Equation 1, and with a decay rate defined byEquation 3. The first cycle of oscillation begins at voltage level V₀and V_(LC) continues to decay to zero as controlled by the Q factor andω. The example illustrated in FIG. 6 represents a typical open orunloaded response when no object is present or proximate to the chargingpad. In FIG. 6 the value of the Q factor is assumed to be 20.

FIG. 7 illustrates a second example in which a response 700 to a passiveping decays according to Equation 3. After the excitation pulse attime=0, the voltage and/or current is seen to oscillate at the resonantfrequency defined by Equation 1, and with a decay rate defined byEquation 3. The first cycle of oscillation begins at voltage level V₀and V_(LC) continues to decay to zero as controlled by the Q factor andω. The example illustrated in FIG. 7 represents a loaded response whenan object is present or proximate to the charging pad loads the coil. InFIG. 7 the Q factor may have a value of 7. V_(LC) oscillates at a higherin the response 700 with respect to the response 600.

FIG. 8 illustrates a set of examples in which differences in responses800, 820, 840 may be observed. A passive ping is initiated when a drivercircuit 504 excites the resonant circuit 506 using a pulse that isshorter than 2.5 μs. Different types of wireless receivers and foreignobjects placed on the transmitter result in different responsesobservable in the voltage at the LC node 510 or current in the resonantcircuit 506 of the transmitter. The differences may indicate variationsin the Q factor of the resonant circuit 506 frequency of the oscillationof V₀. Table 1 illustrates certain examples of objects placed on thecharging pad in relation to an open state.

TABLE 1 50% Decay Q Object Frequency V_(peak) (mV) Cycles Factor Nonepresent 96.98 kHz 134 mV 4.5 20.385 Type-1 Receiver 64.39 kHz  82 mV 3.515.855 Type-2 Receiver 78.14 kHz  78 mV 3.5 15.855 Type-3 Receiver 76.38kHz 122 mV 3.2 14.496 Misaligned 210.40 kHz  110 mV 2.0 9.060 Type-3Receiver Ferrous object 93.80 kHz 110 mV 2.0 9.060 Non-ferrous object100.30 kHz  102 mV 1.5 6.795In Table 1, the Q factor may be calculated as follows:

$\begin{matrix}{{Q = {\frac{\pi N}{\ln(2)} \cong {{4.5}3N}}},} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$where N is the number of cycles from excitation until amplitude fallsbelow 0.5 V₀.

Selectively Activating Coils

According to certain aspects disclosed herein, transmitting coils in oneor more charging cells may be selectively activated to provide anoptimal electromagnetic field for charging a compatible device. In someinstances, transmitting coils may be assigned to charging cells, andsome charging cells may overlap other charging cells. In the latterinstances, the optimal charging configuration may be selected at thecharging cell level. In other instances, charging cells may be definedbased on placement of a device to be charged on a charging surface. Inthese other instances, the combination of coils activated for eachcharging event can vary. In some implementations, a charging device mayinclude a driver circuit that can select one or more cells and/or one ormore predefined charging cells for activation during a charging event.

FIG. 9 illustrates a first topology 900 that supports matrixmultiplexing switching for use in a wireless charger adapted inaccordance with certain aspects disclosed herein. The wireless chargermay select one or more charging cells 100 to charge a receiving device.Charging cells 100 that are not in use can be disconnected from currentflow. A relatively large number of charging cells 100 may be used in thehoneycomb packaging configuration illustrated in FIG. 2 requiring acorresponding number of switches. According to certain aspects disclosedherein, the charging cells 100 may be logically arranged in a matrix 908having multiple cells connected to two or more switches that enablespecific cells to be powered. In the illustrated topology 900, atwo-dimensional matrix 908 is provided, where the dimensions may berepresented by X and Y coordinates. Each of a first set of switches 906is configured to selectively couple a first terminal of each cell in acolumn of cells to a wireless transmitter and/or receiver circuit 902that provide current to activate coils during wireless charging. Each ofa second set of switches 904 is configured to selectively couple asecond terminal of each cell in a row of cells to the wirelesstransmitter and/or receiver circuit 902. A cell is active when bothterminals of the cell are coupled to the wireless transmitter and/orreceiver circuit 902.

The use of a matrix 908 can significantly reduce the number of switchingcomponents needed to operate a network of tuned LC circuits. Forexample, N individually connected cells require at least N switches,whereas a two-dimensional matrix 908 having N cells can be operated with√N switches. The use of a matrix 908 can produce significant costsavings and reduce circuit and/or layout complexity. In one example, a9-cell implementation can be implemented in a 3×3 matrix 908 using 6switches, saving 3 switches. In another example, a 16-cellimplementation can be implemented in a 4×4 matrix 908 using 8 switches,saving 8 switches.

During operation at least 2 switches are closed to actively couple onecoil to a wireless transmitter and/or receiver circuit 902. Multipleswitches can be closed at once in order to facilitate connection ofmultiple coils to the wireless transmitter and/or receiver circuit 902.Multiple switches may be closed, for example, to enable modes ofoperation that drive multiple transmitting coils when transferring powerto a receiving device.

FIG. 10 illustrates a second topology 1000 in which each coil orcharging cell is individually and/or directly driven by a driver circuit1002 in accordance with certain aspects disclosed herein. The drivercircuit 1002 may be configured to select one or more coils or chargingcells 100 from a group of coils 1004 to charge a receiving device. Itwill be appreciated that the concepts disclosed here in relation tocharging cells 100 may be applied to selective activation of individualcoils or stacks of coils. Charging cells 100 that are not in use receiveno current flow. A relatively large number of charging cells 100 may bein use and a switching matrix may be employed to drive individual coilsor groups of coils. In one example, a first switching matrix mayconfigure connections that define a charging cell or a group of coils tobe used during a charging event and a second switching matrix (see,e.g., FIG. 9) may be used to activate the charging cell and/or a groupof selected coils.

Detecting Device Removal From A Multi-Coil Wireless Charger

With reference now to FIG. 11, a multi-coil wireless charging system1100 provided in accordance with certain aspects of this disclosure canbe configured to reliably detect removal of a receiving device 1106while charging is in progress. Arbitrary and/or unanticipated removal ofthe receiving device can cause damage to other receiving devices 1108,in addition to potential loss of detection efficiency for an approachingdevice 1108. The multi-coil wireless charging system 1100 provides acharging surface 1102 that includes multiple transmitting coils 1104₁-1104 _(n). In the illustrated example, a receiving device 1106 isremoved while receiving a charging flux from the n^(th) transmittingcoil (transmitting coil 1104 _(n)).

In some instances, the charging surface 1102 continues to provide acharging current to the transmitting coil 1104 _(n) after the receivingdevice 1106 has been removed. The approaching device 1108 may be placedon the charging surface 1102 while the charging current is flowing. Thecharging current is typically configured based on the capabilities ofthe receiving device 1106, which may differ from the capabilities of theapproaching device 1108. Damage to the approaching device 1108 can occurif the approaching device 1108 is not designed to handle the level ofinduced current intended for the original receiving device 1106.

Certain aspects of this disclosure enable the multi-coil wirelesscharging system 1100 to rapidly and reliably detect the removal of thereceiving device 1106 from the charging surface 1102. The multi-coilwireless charging system 1100 may discontinue the flow of the chargingcurrent to the active transmitting coil 1104 _(n) upon detecting theremoval of the receiving device 1106. The multi-coil wireless chargingsystem 1100 may configure the charging surface 1102 to detect objects,including the approaching device 1108 upon detecting the removal of thereceiving device 1106 and discontinuation of the charging current.

According to certain aspects of the disclosure, removal of the receivingdevice 1106 may be detected by monitoring charging circuits, or certaincharacteristics of one or more of the transmitting coils 1104 ₁-1104_(n). In certain examples, the removal of the receiving device 1106 maybe detected based on changes in measured electrical quantities that canbe attributed to changes in electromagnetic coupling between thetransmitting coil 1104 _(n) and a receiving coil in the receiving device1106.

In one example, Dynamic Inferred Coupling Estimation (DICE) may be usedto detect quality of coupling in real-time. DICE may include anevaluation of the ratio of real power to reactive power in a circuitthat includes a transmitting coil and series resonant capacitor. Theamount of reactive power stored in the inductor-capacitor (LC) circuitof the transmitter is substantially influenced by the couplingcoefficient. The coupling coefficient defines the ratio of mutualinductance to leakage inductance in the LC circuit of the wirelesstransmitter. For example, leakage inductance in the LC circuit of thewireless transmitter may be expressed as:Tx _(leakage) =L _(Tx)×(1−k),  (Eq. 3)where L_(Tx) represents the self-inductance of the transmitter coil, andk represents the coupling coefficient. Decreasing coupling reducescoupling coefficient and increases leakage inductance, resulting in morereactive energy being stored in the leakage inductance of thetransmitter. Energy stored in the leakage inductance does not contributeto power transfer and, as energy builds up in the leakage inductance,the voltage at the LC node increases.

Certain aspects of the coupling between one or more transmitting coils1104 ₁-1104 _(n) and a receiving device 1106 may be characterized byvoltage measured at the LC node. Voltage measurements taken at the LCnode may be available for other reasons. In some instances, voltage atthe LC node may be monitored as an overvoltage indicator used to protectpower electronics and the resonant capacitor. In one example, themeasurement circuit includes a voltage comparator configured to detectvoltages exceeding a threshold level. According to certain aspectsdisclosed herein, a measurement circuit may be added, or an existingmeasurement circuit may be used to quantify or compare a voltage at theLC node that varies directly with the quality of coupling.

FIG. 12 is a graphical representation 1200 of certain aspects of adevice removal event that may be monitored in accordance with certainaspects disclosed herein. Two curves 1202, 1204 represent the state ofelectrical quantities that are measurable in the multi-coil wirelesscharging system 1100.

A first curve 1202 represents the magnitude of the current flowing toone or more active transmitting coils 1104 ₁-1104 _(n) to charge areceiving device 1106. The receiving device 1106 is initially placed inproximity to the charging surface 1102 and is wirelessly receivingpower. The receiving device 1106 then begins to move away from thecharging surface 1102, commencing at a first point in time 1206 (t₁),until the receiving device 1106 is receiving no power or aninsignificant level of power from the active transmitting coils 1104₁-1104 _(n) at a second point in time 1208 (t₂). As illustrated in FIG.12, the charging current can be expected to drop off when the receivingdevice 1106 is removed. The first curve 1202 includes a step between theinitial level of the charging current and the level of the chargingcurrent or quiescent current after the receiving device 1106 has beenremoved. A sharp drop-off in charging current may be observed, even whenthe receiving device 1106 is removed at moderate rate, given theinverse-square relationship associated with electromagnetic couplingwhen the distance between transmitter and receiver is increasing.

A second curve 1204 represents magnitude of the tank voltage measured atan LC node in a resonant circuit that includes one or more activetransmitting coils 1104 ₁-1104 _(n) used to wirelessly charge areceiving device 1106. The receiving device 1106 that is initiallyplaced in proximity to the charging surface 1102 and is wirelesslyreceiving power begins to be move away from the charging surface 1102,commencing at the first point in time 1206 (t₁), and continues movingaway until the receiving device 1106 is receiving no power or aninsignificant power from the active transmitting coils 1104 ₁-1104 _(n)at the second point in time 1208 (t₂). As illustrated in FIG. 12, thetank voltage can be expected to increase with the impedance of theresonant circuit that results from the receiving device 1106 beingremoved. The second curve 1204 includes a step between the initial levelof the tank voltage and the level of the tank voltage after thereceiving device 1106 has been removed. A sharp increase in impedanceand tank voltage may be observed, even when the receiving device 1106 isremoved at moderate rate, given the inverse-square relationshipassociated with electromagnetic coupling when the distance betweentransmitter and receiver is increasing.

In accordance with certain aspects of this disclosure, charging currentprovided to the one or more active transmitting coils 1104 ₁-1104 _(n)and/or the tank voltage may be monitored during power transfers. Thecharging current may be discontinued when a step in current or voltageexceeds a threshold difference value or when the rate of change incurrent (di/dt) or voltage (dv/dt) exceeds a threshold rate of change.The threshold difference value and/or the threshold rate of change maybe preconfigured by application, during system initialization and/orduring manufacture or assembly. In some implementations the thresholddifference value and/or the threshold rate of change may be dynamicallyconfigured based on a charging configuration identifying a number oftransmitting coils 1104 ₁-1104 _(n) to be used for wireless, chargingthe size of the charging current, and/or the structure or internalconfiguration of the receiving device 1106.

In some instances, variability in charging current or tank voltage maybe observed when the receiving device 1106 is not being removed. Forexample, charging current or tank voltage may vary due to vibration orslippage of the receiving device 1106 or charging surface 1102, physicalinstability caused by motion of the multi-coil wireless charging system1100, or due to variations in temperature or drift in power supplyoutput. Certain implementations may employ low-pass filtering toaccommodate such variability in charging current or tank voltage.

FIG. 13 illustrates a filtered threshold detection circuit 1300 thatemploys low-pass filtering to accommodate variability in chargingcurrent or tank voltage that is not attributable to movement of thereceiving device 1106. FIG. 13 includes a graph 1320 that illustratescertain aspects related to the operation of the filtered thresholddetection circuit 1300. In the illustrated example, the filteredthreshold detection circuit 1300 receives an input signal 1310representative of the charging current flowing to one or more activetransmitting coils 1104 ₁-1104 _(n) used to charge the receiving device1106. The input signal 1310 is provided to a low-pass filter 1302 thatcan delay step changes in the input signal 1310, and/or that slows therate of change in the input signal 1310. A comparison circuit 1304compares the output 1312 of the low-pass filter 1302 with a thresholdsignal 1314 generated from the input signal 1310 using a scaling factor1308 or offset that is based on the prior state of the output 1316 ofthe comparison circuit 1304. The threshold signal 1314 may be generatedby a feedback circuit 1306 to provide hysteresis in the filteredthreshold detection circuit 1300. The threshold signal 1314 provides areference point 1330 that enables the comparison circuit 1304 toreliably indicate removal of the receiving device 1106. The low-passfilter 1302 may be configured with a filter constant configured suchthat normal small variations in current 1332 do not cause a deviceremoval indication.

A first curve 1322 represents the magnitude of a current flowing to oneor more active transmitting coils 1104 ₁-1104 _(n) to charge a receivingdevice 1106. The receiving device 1106 is initially placed in proximityto the charging surface 1102 and is wirelessly receiving power. A secondcurve 1324 represents the threshold value used to determine when a stepchange in charging current indicates a device removal event. Thereceiving device 1106 begins to move away from the charging surface1102, commencing at a first point in time 1326 (t₁), until the receivingdevice 1106 is receiving no power or an insignificant power from theactive transmitting coils 1104 ₁-1104 _(n), at a second point in time1328 (t₂).

The charging current drops off when the receiving device 1106 is removedresulting in a step between the initial level of the charging currentand the level of the charging current or quiescent current after thereceiving device 1106 has been removed. The threshold signal 1314 canensure that large step changes in the charging current (or largeincreases in tank voltage) is sufficient to cross the threshold value.

In another aspect of the disclosure, slot-based techniques may be usedto enable detection of removal of a receiving device 1106. In oneexample, a time-slot is provided during which charging current issuspended a short period of time to enable one or more measurementsand/or the interrogation of one or more sensors.

FIG. 14 illustrates a Q-factor comparison circuit 1400 and correspondingtiming diagram 1420 that illustrate detection of removal of a receivingdevice 1106 during a measurement slot 1424. The timing diagram 1420includes a curve 1422 representing magnitude of a charging currentflowing in one or more active transmitting coils 1104 ₁-1104 _(n) when acharging surface 1102 of a multi-coil wireless charging system 1100 isconfigured to charge a receiving device 1106. The measurement slot 1424may be provided periodically or in response to detection of a stepchange in the magnitude of the charging current or tank voltage. Aslotted Q-factor test may be performed during the measurement slot 1424.The measurement slot 1424 may be provided when the multi-coil wirelesscharging system 1100 suspends or terminates the charging current. In oneexample, the measurement slot 1424 has a duration of up to 100microseconds (μs). Energy stored in the resonant circuit decays at ratethat is determined in part by the Q-factor of the resonant circuit. TheQ-factor of the resonant circuit may be used as a measure of theelectromagnetic coupling between certain active transmitting coils 1104₁-1104 _(n) in the charging surface 1102 and the receiving coil in thereceiving device 1106.

A slotted Q-factor test may commence at a first point in time 1426 (t₁),when the multi-coil wireless charging system 1100 ceases to power theresonant circuit that includes the active transmitting coils 1104 ₁-1104_(n). The magnitude of the current in the resonant circuit decays 1428at a rate determined by the Q-factor of the resonant circuit. In oneexample, a Q-factor 1402 may be calculated and compared to a referenceQ-factor 1404 using a comparator 1406. The reference Q-factor 1404 maycorrespond to a Q-factor calculated when no receiving device iselectromagnetically coupled to the active transmitting coils 1104 ₁-1104_(n).

In some implementations, the filtered threshold detection circuit 1300may be used to compare calculated Q-factor with a threshold Q-factor. Insome implementations, a measurement slot 1424 is provided periodicallyto enable the slotted Q-factor test to be used to detect presence offoreign objects as well as device removal events.

Another aspect of this disclosure relates to detecting the removal of areceiving device 1106 using threshold values and other parameters thatare maintained in lookup tables. For example, lookup tables may be usedto maintain measured values of charging current, tank voltage, Q-factorand other characteristics of a multi-coil wireless charging system 1100.In some implementations, the lookup tables may maintain threshold valuesand other parameters for different charging configurations. Eachcharging configuration may define a set of transmitting coils 1104₁-1104 _(n) to be used for charging the receiving device 1106 and acurrent distribution among the transmitting coils 1104 ₁-1104 _(n). Forexample, one or more charging configurations may define phase offsetsfor currents provided to different transmitting coils 1104 ₁-1104 _(n)when an electromagnetic flux is to be concentrated at a specificlocation or directed within an area spanned by the transmitting coils1104 ₁-1104 _(n). One or more charging configurations may be provided tomatch the capabilities, location, orientation, charging state, and/oranother characteristic of the receiving device 1106. The use of lookuptables can improve the efficiency of detection circuits and processesused to determine when a receiving device 1106 has been removed.

FIG. 15 includes graphs 1500, 1520 illustrating the use of look-uptables for detecting a device removal event that may be monitored inaccordance with certain aspects disclosed herein. In one example, alookup table (LUT) may maintain information identifying a knownquiescent or “empty” power and/or a known current draw for a chargingconfiguration. The multi-coil wireless charging system 1100 may comparemeasured power, voltage and/or current after a step event has beendetected with a threshold value or other corresponding value forquiescent power, voltage and/or current draw maintained in a lookuptable. The comparison may indicate whether the step event corresponds toa load change or a device removal. A load change may occur when areceiving device dumps its load. The lookup table may include values forquiescent power, voltage and/or current measured for one or moretransmitting coils 1104 ₁-1104 _(n) that are not electromagneticallycoupled to a receiving coil or another object that may affect theresonant frequency or Q-factor of the resonant circuit that includes themore transmitting coils 1104 ₁-1104 _(n). In some implementations, thelookup table may be populated with values measured for differentcharging configurations. In some implementations, the lookup table maybe populated during a system configuration or calibration procedure.

The first graph 1500 illustrates an example in which the thresholds1506, 1508 identified in a lookup table can reliably indicate that thereceiving device 1106 has been removed. The second graph 1520illustrates an example in which the thresholds 1526, 1528 maintained ina lookup table can reliably indicate that a load change has occurred. Afirst curve 1502, 1522 represents the magnitude of the current flowingto one or more active transmitting coils 1104 ₁-1104 _(n) to charge areceiving device 1106. The receiving device 1106 is initially placed inproximity to the charging surface 1102 and is wirelessly receivingpower. The receiving device 1106 then begins to move away from thecharging surface 1102, commencing at a first point in time 1512, 1532(t₁), until the receiving device 1106 is receiving reduced power,corresponding to a drop-off in the charging current. Each of the firstcurves 1502, 1522 includes a step 1510, 1530 between the initial levelof the charging current and the level of the charging current after thereceiving device 1106 has been removed.

In one aspect of the disclosure, the magnitude of the charging currentmeasured after the step 1510, 1530 is compared to a current threshold1508, 1528 (or reference quiescent current level) obtained from a lookuptable. In one example, the multi-coil wireless charging system 1100 mayterminate the charging current based on the difference between thecharging current level after the step 1510, 1530 and a referencequiescent current level or current threshold 1508, 1528. In the exampleillustrated by the first graph 1500, the multi-coil wireless chargingsystem 1100 may terminate the charging current when the charging currentlevel is within a configured range that includes a reference quiescentcurrent level or is less than a current threshold 1508, 1528 calculatedusing the reference quiescent current level. In the example illustratedby the second graph 1520, the multi-coil wireless charging system 1100may continue to provide the charging current when the charging currentlevel is greater than the current threshold 1508, 1528 by an amount thatindicates a load change event has occurred.

A second curve 1504, 1524 in the graphs 1500, 1520 represents themagnitude of the tank voltage measured across a resonant circuit thatincludes to one or more active transmitting coils 1104 ₁-1104 _(g). Thereceiving device 1106 is initially placed in proximity to the chargingsurface 1102 and is wirelessly receiving power. The receiving device1106 then begins to move away from the charging surface 1102, commencingat a first point in time 1512, 1532 (t₁), until the receiving device1106 is receiving reduced power, corresponding to an increase in thetank voltage. Each of the second curves 1504, 1524 includes a step 1510,1530 between the initial level of the tank voltage and the level of thetank voltage after the receiving device 1106 has been removed.

In one aspect of the disclosure, the magnitude of the tank voltagemeasured after the step 1510, 1530 is compared to a reference quiescenttank voltage or a voltage threshold 1506, 1526 obtained from a lookuptable. The multi-coil wireless charging system 1100 may terminate thecharging current based on the difference between the tank voltage levelafter the step 1510, 1530 and the reference quiescent tank voltage orthe voltage threshold 1506, 1526. In the example illustrated by thefirst graph 1500, the multi-coil wireless charging system 1100 mayterminate the charging current when the tank voltage level is within aconfigured range that includes the quiescent tank voltage or is greaterthan the voltage threshold 1506, 1526. In the example illustrated by thesecond graph 1520, the multi-coil wireless charging system 1100 maycontinue to provide the charging current when the tank voltage is lessthan the voltage threshold 1506, 1526 indicating that a load changeevent has occurred.

FIG. 16 is a flowchart 1600 that illustrates an example of a procedurebased on the examples illustrated in FIG. 15. The procedure may beperformed at a multi-coil wireless charging system 1100. At block 1602,the multi-coil wireless charging system 1100 may begin providing acharging current to a receiving device 1106 in accordance with acharging configuration. The multi-coil wireless charging system 1100 maycontinue charging until at block 1604, the multi-coil wireless chargingsystem 1100 detects a step change in a measured value. In one example,the measured value may represent the magnitude of the charging current.In another example, the measured value may represent a tank voltage. Atblock 1606, the multi-coil wireless charging system 1100 may measure thevalue after the step. At block 1608, the multi-coil wireless chargingsystem 1100 may compare the measured value to a threshold stored in alookup table. The threshold may be calculated from an idle or quiescentvalue. The relationship between the measured value and the threshold mayindicate whether the step change in the measured value is the result ofremoval of the receiving device 1106. When the multi-coil wirelesscharging system 1100 determines at block 1610 that the step changerelates to a device removal event, then at block 1612, the multi-coilwireless charging system 1100 may terminate the charging current. If themulti-coil wireless charging system 1100 determines at block 1610 thatthe step change does not relate a device removal event, then the processmay continue at block 1604.

FIG. 17 is a graph 1700 that illustrates the use of measured quiescentpower draw or a preconfigured, or premeasured idle transfer power drawvalue maintained in a look-up table for detecting a device removal eventin accordance with certain aspects disclosed herein. In one aspect, ameasurement of power transfer is obtained during an initialconfiguration interval period 1702 that may be associated with a pingprocedure. The curve 1710 represents power or current transfer from themulti-coil wireless charging system 1100 to the receiving device 1106.In one example, a measured power transfer value that characterizes aminimal or quiescent power transfer state may be used to set a knownoperating point for the multi-coil wireless charging system 1100. Theknown operating point may be used to define a threshold for detectingdevice removal. The latter threshold may be referred to as the MeasuredThreshold 1716 herein. In another aspect, a threshold for detectingdevice removal may be obtained from a lookup table. The latter thresholdmay be referred to as the LUT Threshold 1718 herein. The LUT Threshold1718 may be calculated or measured during system initialization,assembly or during a calibration procedure. In one example, the LUTThreshold 1718 may be calculated or measured when no chargeable deviceor other object is located on or near the charging surface 1102.

The curve 1710 may correspond to a charging current that enables powerto be transferred from the multi-coil wireless charging system 1100 tothe receiving device 1106. After initial detection and/or configurationof the receiving device 1106, a minimal power transfer level 1712 may bedetermined and/or used to set the Measured Threshold 1716. A powertransfer period 1704 ensues. The power transfer period 1704 continuesuntil an event 1706 is detected, where the level of power transferexhibits a step drop. In the illustrated example, the level of powertransfer drops to a lower level 1714 that may be above or below thethreshold used to determine device removal. The threshold may beselected from Measured Threshold 1716 or LUT Threshold 1718. Themulti-coil wireless charging system 1100 may initiate a measurement slot1708 in order to establish or confirm a device removal has occurred.During the measurement slot 1708, the multi-coil wireless chargingsystem 1100 may measure and compare quiescent power draw to the selectedthreshold. The multi-coil wireless charging system 1100 may discontinuethe charging current after determining that the receiving device 1106has been removed. The multi-coil wireless charging system 1100 maycontinue the power transfer at the lower level 1714 after determiningthat the receiving device 1106 has not been removed.

FIG. 18 is a flowchart 1800 that illustrates a first example of aprocedure for device removal detection based on measured quiescent powerdraw. The procedure may be performed at a multi-coil wireless chargingsystem 1100. At block 1802, the multi-coil wireless charging system 1100may detect the presence of a receiving device 1106 that has been placedon or near the charging surface 1102. During an initial configurationinterval period 1702, the multi-coil wireless charging system 1100 mayinterrogate and/or negotiate with the receiving device 1106 to generatea charging configuration. At block 1804, the multi-coil wirelesscharging system 1100 may provide a quiescent current to one or moreactive transmitting coils 1104 ₁-1104 _(n) and may measure quiescentpower draw. The multi-coil wireless charging system 1100 may use themeasured quiescent power draw to establish a Measured Threshold 1716. Inone example, the Measured Threshold 1716 may be stored in non-volatilememory such as random-access memory (RAM) or register-based memory.

The power transfer period 1704 begins, during which the multi-coilwireless charging system 1100 may provide a charging current to theactive transmitting coils 1104 ₁-1104 _(n) to enable power transfer to areceiving device 1106 in accordance with the charging configuration. Themulti-coil wireless charging system 1100 may continue charging until atblock 1806, the multi-coil wireless charging system 1100 detects a stepchange in a measured power draw. In one example, the measured power drawmay be represented by the magnitude of the charging current. In anotherexample, the measured power draw may be represented by a tank voltage.At block 1808, the multi-coil wireless charging system 1100 may providea measurement slot 1708 during which the charging current is reduced toquiescent levels. The multi-coil wireless charging system 1100 maymeasure the quiescent power draw during the measurement slot 1708. Atblock 1810, the multi-coil wireless charging system 1100 may compare themeasured quiescent power draw to the Measured Threshold 1716. Therelationship between the measured quiescent power draw and the MeasuredThreshold 1716 may indicate whether the step change in the power draw isthe result of removal of the receiving device 1106. When the multi-coilwireless charging system 1100 determines at block 1812 that the stepchange relates to a device removal event, then at block 1814, themulti-coil wireless charging system 1100 may terminate the chargingcurrent. If the multi-coil wireless charging system 1100 determines atblock 1812 that the step change does not relate a device removal event,then the process may continue at block 1806.

FIG. 19 is a flowchart 1900 that illustrates a second example of aprocedure for device removal detection based on measured quiescent powerdraw. The procedure may be performed at a multi-coil wireless chargingsystem 1100. At block 1902, the multi-coil wireless charging system 1100may detect the presence of a receiving device 1106 that has been placedon or near the charging surface 1102. During an initial configurationinterval period 1702, the multi-coil wireless charging system 1100 mayinterrogate and/or negotiate with the receiving device 1106 to generatea charging configuration.

The power transfer period 1704 begins, during which the multi-coilwireless charging system 1100 may provide a charging current to one ormore active transmitting coils 1104 ₁-1104 _(n) configured to wirelesslytransfer power to a receiving device 1106 in accordance with thecharging configuration. The multi-coil wireless charging system 1100 maycontinue charging until at block 1904, the multi-coil wireless chargingsystem 1100 detects a step change in a measured power draw. In oneexample, the measured power draw may be represented by the magnitude ofthe charging current. In another example, the measured power draw may berepresented by a tank voltage. At block 1906, the multi-coil wirelesscharging system 1100 may provide a measurement slot 1708 during whichthe charging current is reduced to quiescent levels. The multi-coilwireless charging system 1100 may measure the quiescent power drawduring the measurement slot 1708. At block 1908, the multi-coil wirelesscharging system 1100 may compare the measured quiescent power draw to anLUT Threshold 1718. The LUT Threshold 1718 may be premeasured orprecalculated based on a quiescent (empty-coil) power draw. Thepremeasured or precalculated power draw may be maintained in a lookuptable stored in non-volatile memory, such as flash memory. Therelationship between the measured quiescent power draw to the LUTThreshold 1718 may indicate whether the step change in the power draw isthe result of removal of the receiving device 1106. When the multi-coilwireless charging system 1100 determines at block 1910 that the stepchange relates to a device removal event, then at block 1912, themulti-coil wireless charging system 1100 may terminate the chargingcurrent. If the multi-coil wireless charging system 1100 determines atblock 1910 that the step change does not relate a device removal event,then the process may continue at block 1904.

FIG. 20 is a graph 2000 that illustrates the use of a measurement slotto perform a ping procedure that can determine whether the receivingdevice 1106 remains on or near the charging surface 1102. The pingprocedure may include an active and/or passive ping. The ping proceduremay include an analog and/or digital ping. An initial configurationinterval period 2002 may be provided after a device or object has beendetected on or near the charging surface 1102. A ping procedure may beconducted within the initial configuration interval period 2002 todetermine whether the detected object is a chargeable object and todetermine a charging configuration suitable for a chargeable object.

The curve 2010 represents power transfer from the multi-coil wirelesscharging system 1100 to the receiving device 1106. During a powertransfer period 2004, an event 2006 may be detected, where the level ofpower transfer exhibits a step drop. The multi-coil wireless chargingsystem 1100 may initiate a measurement slot 2008 in order to establishor confirm a device removal event. During the measurement slot 2008, themulti-coil wireless charging system 1100 may terminate the chargingcurrent to permit a ping procedure to be used to determine whether thereceiving device 1106 has been removed. The multi-coil wireless chargingsystem 1100 may discontinue the charging current after determining thatthe receiving device 1106 has been removed. The multi-coil wirelesscharging system 1100 may continue power transfer at a lower level 2012after determining that the receiving device 1106 has not been removed.

FIG. 21 is a flowchart 2100 that illustrates an example of a method fordevice removal detection based on a ping procedure performed during ameasurement slot. The method may be performed at a multi-coil wirelesscharging system 1100. At block 2102, the multi-coil wireless chargingsystem 1100 may detect the presence of a receiving device 1106 that hasbeen placed on or near the charging surface 1102. During an initialconfiguration interval period 2002, the multi-coil wireless chargingsystem 1100 may interrogate and/or negotiate with the receiving device1106 to generate a charging configuration.

The power transfer period 2004 begins, during which the multi-coilwireless charging system 1100 may provide a charging current to one ormore active transmitting coils 1104 ₁-1104 _(n) configured to wirelesslytransfer power to a receiving device 1106 in accordance with thecharging configuration. The multi-coil wireless charging system 1100 maycontinue charging until at block 2104, the multi-coil wireless chargingsystem 1100 detects a step change in a measured power draw, current ortank voltage. At block 2106, the multi-coil wireless charging system1100 may provide a measurement slot 2008 during which one or more pingprocedures may be executed to determine whether the step change is theresult of removal of the receiving device 1106. When the multi-coilwireless charging system 1100 determines at block 2108 that the stepchange relates to a device removal event, then at block 2110, themulti-coil wireless charging system 1100 may terminate the chargingcurrent. If the multi-coil wireless charging system 1100 determines atblock 2108 that the step change does not relate a device removal event,then the process may continue at block 2104.

According to certain aspects of this disclosure, presence, positionand/or orientation of a receiving device may be determined using alocation sensing technique that involves, for example, detectingdifferences or changes in capacitance, resistance, inductance, touch,pressure, temperature, load, strain, and/or another appropriate type ofsensing. Location sensing may be employed to determine presence orlocation of an object or device to be charged. Location sensing may alsobe employed to detect removal of a receiving device during powertransfer from a charging surface.

FIG. 22 illustrates a first example of a charging surface 2200 of awireless charger that includes one or more sensors 2202 that can detectremoval of a receiving device during power transfer from the chargingsurface 2200. In this example, the sensors 2202 may include capacitive,inductive, or hall effect sense elements configured to detect thepresence of a device. In some implementations, the sense elements mayborder the charging coils (LP1-LP18) provided in the charging surface2200. In some implementations, the sense elements may border individualcharging coils or groups of charging coils. In certain implementations,charging zones may be identified on the charging surface 2200, and thesense elements may define or monitor the outer limits of each chargingzone.

The sensors 2202 may also be used to detect changes indicative ofremoval of a receiving device from the charging surface 2200. In someimplementations, the sensors 2202 may support or enhance removaldetection techniques based on measurements of charging current, tankvoltages and/or power draw. The use of sensors 2202 may improvereliability, efficiency and can reduce power consumption and processorloading.

FIG. 23 illustrates a second example of a charging surface 2300 of awireless charger that includes one or more sensors 2312 ₁-2312 _(n)and/or 2314 ₁-2314 _(n) that can be used for detecting device removal.The sensors 2312 ₁-2312 _(n) and/or 2314 ₁-2314 _(n) can measuredeformation, loading and/or weight attributable to a device or objectplaced on or near the charging surface 2300. The sensors 2312 ₁-2312_(n) and/or 2314 ₁-2314 _(n) may be configured to measure deformation asmechanical strain which may quantify the displacement between two pointson a surface. In one example, sensors 2312 ₁-2312 _(n) placed betweentransmitter coils 2304 ₁-2304 _(n) and a circuit board 2302 may providemeasurements that correspond to the combined weight of the transmittercoils 2304 ₁-2304 _(n) and the device or object placed on or near thecharging surface 2300. The weight of the device or object may becalculated from the combined weight, or a change in the combined weightmay be used to indicate placement or removal of the device or object. Inanother example, sensors 2314 ₁-2314 _(n) placed on the exterior surfaceof the charging surface 2300 and above the transmitter coils 2304 ₁-2304_(n) may provide measurements that correspond to the deformation of theexterior surface caused by the weight and shape of the object placed onor near the charging surface 2300.

The sensors 2312 ₁-2312 _(n) and/or 2314 ₁-2314 _(n) may be used todetect changes indicative of removal of a receiving device 2306 from thecharging surface 2300. In some implementations, the sensors 2312 ₁-2312_(n) and/or 2314 ₁-2314 _(n) may support or enhance removal detectiontechniques based on measurements of charging current, tank voltagesand/or power draw. The use of sensors 2312 ₁-2312 _(n) and/or 2314₁-2314 _(n) may improve reliability, efficiency and can reduce powerconsumption and processor loading.

FIG. 24 illustrates a third example of a charging surface 2400 of awireless charger that includes one or more sensors 2412 ₁-2412 _(n) usedfor detecting device removal. The sensors 2412 ₁-2412 _(n) can measuresmall changes in movement or vibration caused when a receiving device2406 or other object is picked up or otherwise removed from the chargingsurface 2400. In one example, sensors 2412 ₁-2412 _(n) are placedbetween transmitter coils 2404 ₁-2404 _(n) and a circuit board 2402.

In some implementations, the sensors 2412 ₁-2412 _(n) may support orenhance removal detection techniques based on measurements of chargingcurrent, tank voltages and/or power draw. The use of sensors 2412 ₁-2412_(n) may improve reliability, efficiency and can reduce powerconsumption and processor loading.

FIG. 25 illustrates a fourth example of a charging surface 2500 of awireless charger that includes one or more devices 2504 ₁-2504 ₄ and/or2506 ₁-2506 ₄ that may be used for detecting device removal. The devices2504 ₁-2504 ₄ and/or 2506 ₁-2506 ₄ may include infrared and/orultrasonic transmitting and sensing devices located co-planar with anexterior surface of the charging surface 2500. In the illustratedexample, the transmitting devices 2504 ₁-2504 ₄ direct infrared orultrasonic beams to set of sensing devices 2506 ₁-2506 ₄. One or morebeams may be interrupted when a receiving device 2502 is placed on thecharging surface 2500 between corresponding pairs of the devices 2504₁-2504 ₄ and/or 2506 ₁-2506 ₄. Removal of the receiving device 2502enables one or more of the sensing devices 2506 ₁-2506 ₄ to detect atransmitted beam.

In some implementations, the devices 2504 ₁-2504 ₄ and/or 2506 ₁-2506 ₄may support or enhance removal detection techniques based onmeasurements of charging current, tank voltages and/or power draw. Theuse of the devices 2504 ₁-2504 ₄ and/or 2506 ₁-2506 ₄ may improvereliability, efficiency and can reduce power consumption and processorloading. In some implementations, increased numbers of transmittingand/or sensing devices 2504 ₁-2504 ₄ and/or 2506 ₁-2506 ₄ can provideimproved resolution of device location that may be expressed in X and Ycoordinates.

FIG. 26 illustrates a fifth example of a charging surface 2600 of awireless charger that includes one or more sensing devices 2604 ₁-2604 ₅that may be used for detecting device removal. The sensing devices 2604₁-2604 ₅ may include infrared and/or ultrasonic combined transmittersand sensors. The sensing devices 2604 ₁-2604 ₅ may be located co-planarwith an exterior surface of the charging surface 2600. In theillustrated example, sensing devices 2604 ₁-2604 ₅ transmit an infraredor ultrasonic beam and are configured to sense characteristics ofreflections of the beams. One or more beams may be reflected by areceiving device 2602 placed on the charging surface 2600. The sensingdevices 2604 ₁-2604 ₅ may detect phase changes, angles of reflection andother characteristics of the reflected beams, thereby permittingdetection of the receiving device 2602. Removal of the receiving device2602 eliminates the reflected beams or modifies the characteristics ofthe reflected beams.

In some implementations, the sensing devices 2604 ₁-2604 ₅ may supportor enhance removal detection techniques based on measurements ofcharging current, tank voltages and/or power draw. The use of thesensing devices 2604 ₁-2604 ₅ may improve reliability, efficiency andcan reduce power consumption and processor loading. In someimplementations, increased numbers of sensing devices 2604 ₁-2604 ₅ canprovide improved resolution of device location that may be expressed inX and Y coordinates. In some implementations, the sensing devices 2604₁-2604 ₅ may detect distance between the sensing devices 2604 ₁-2604 ₅and the receiving device 2602, enabling two sensors to determine preciselocations of the receiving device 2602.

Example of a Processing Circuit

FIG. 27 illustrates an example of a hardware implementation for anapparatus 2700 that may be incorporated in a charging device or in areceiving device that enables a battery to be wirelessly charged. Insome examples, the apparatus 2700 may perform one or more functionsdisclosed herein. In accordance with various aspects of the disclosure,an element, or any portion of an element, or any combination of elementsas disclosed herein may be implemented using a processing circuit 2702.The processing circuit 2702 may include one or more processors 2704 thatare controlled by some combination of hardware and software modules.Examples of processors 2704 include microprocessors, microcontrollers,digital signal processors (DSPs), SoCs, ASICs, field programmable gatearrays (FPGAs), programmable logic devices (PLDs), state machines,sequencers, gated logic, discrete hardware circuits, and other suitablehardware configured to perform the various functionality describedthroughout this disclosure. The one or more processors 2704 may includespecialized processors that perform specific functions, and that may beconfigured, augmented or controlled by one of the software modules 2716.The one or more processors 2704 may be configured through a combinationof software modules 2716 loaded during initialization, and furtherconfigured by loading or unloading one or more software modules 2716during operation.

In the illustrated example, the processing circuit 2702 may beimplemented with a bus architecture, represented generally by the bus2710. The bus 2710 may include any number of interconnecting buses andbridges depending on the specific application of the processing circuit2702 and the overall design constraints. The bus 2710 links togethervarious circuits including the one or more processors 2704, and storage2706. Storage 2706 may include memory devices and mass storage devices,and may be referred to herein as computer-readable media and/orprocessor-readable media. The storage 2706 may include transitorystorage media and/or non-transitory storage media.

The bus 2710 may also link various other circuits such as timingsources, timers, peripherals, voltage regulators, and power managementcircuits. A bus interface 2708 may provide an interface between the bus2710 and one or more transceivers 2712. In one example, a transceiver2712 may be provided to enable the apparatus 2700 to communicate with acharging or receiving device in accordance with a standards-definedprotocol. Depending upon the nature of the apparatus 2700, a userinterface 2718 (e.g., keypad, display, speaker, microphone, joystick)may also be provided, and may be communicatively coupled to the bus 2710directly or through the bus interface 2708.

A processor 2704 may be responsible for managing the bus 2710 and forgeneral processing that may include the execution of software stored ina computer-readable medium that may include the storage 2706. In thisrespect, the processing circuit 2702, including the processor 2704, maybe used to implement any of the methods, functions and techniquesdisclosed herein. The storage 2706 may be used for storing data that ismanipulated by the processor 2704 when executing software, and thesoftware may be configured to implement any one of the methods disclosedherein.

One or more processors 2704 in the processing circuit 2702 may executesoftware. Software shall be construed broadly to mean instructions,instruction sets, code, code segments, program code, programs,subprograms, software modules, applications, software applications,software packages, routines, subroutines, objects, executables, threadsof execution, procedures, functions, algorithms, etc., whether referredto as software, firmware, middleware, microcode, hardware descriptionlanguage, or otherwise. The software may reside in computer-readableform in the storage 2706 or in an external computer-readable medium. Theexternal computer-readable medium and/or storage 2706 may include anon-transitory computer-readable medium. A non-transitorycomputer-readable medium includes, by way of example, a magnetic storagedevice (e.g., hard disk, floppy disk, magnetic strip), an optical disk(e.g., a compact disc (CD) or a digital versatile disc (DVD)), a smartcard, a flash memory device (e.g., a “flash drive,” a card, a stick, ora key drive), RAM, ROM, a programmable read-only memory (PROM), anerasable PROM (EPROM) including EEPROM, a register, a removable disk,and any other suitable medium for storing software and/or instructionsthat may be accessed and read by a computer. The computer-readablemedium and/or storage 2706 may also include, by way of example, acarrier wave, a transmission line, and any other suitable medium fortransmitting software and/or instructions that may be accessed and readby a computer. Computer-readable medium and/or the storage 2706 mayreside in the processing circuit 2702, in the processor 2704, externalto the processing circuit 2702, or be distributed across multipleentities including the processing circuit 2702. The computer-readablemedium and/or storage 2706 may be embodied in a computer programproduct. By way of example, a computer program product may include acomputer-readable medium in packaging materials. Those skilled in theart will recognize how best to implement the described functionalitypresented throughout this disclosure depending on the particularapplication and the overall design constraints imposed on the overallsystem.

The storage 2706 may maintain and/or organize software in loadable codesegments, modules, applications, programs, etc., which may be referredto herein as software modules 2716. Each of the software modules 2716may include instructions and data that, when installed or loaded on theprocessing circuit 2702 and executed by the one or more processors 2704,contribute to a run-time image 2714 that controls the operation of theone or more processors 2704. When executed, certain instructions maycause the processing circuit 2702 to perform functions in accordancewith certain methods, algorithms and processes described herein.

Some of the software modules 2716 may be loaded during initialization ofthe processing circuit 2702, and these software modules 2716 mayconfigure the processing circuit 2702 to enable performance of thevarious functions disclosed herein. For example, some software modules2716 may configure internal devices and/or logic circuits 2722 of theprocessor 2704, and may manage access to external devices such as atransceiver 2712, the bus interface 2708, the user interface 2718,timers, mathematical coprocessors, and so on. The software modules 2716may include a control program and/or an operating system that interactswith interrupt handlers and device drivers, and that controls access tovarious resources provided by the processing circuit 2702. The resourcesmay include memory, processing time, access to a transceiver 2712, theuser interface 2718, and so on.

One or more processors 2704 of the processing circuit 2702 may bemultifunctional, whereby some of the software modules 2716 are loadedand configured to perform different functions or different instances ofthe same function. The one or more processors 2704 may additionally beadapted to manage background tasks initiated in response to inputs fromthe user interface 2718, the transceiver 2712, and device drivers, forexample. To support the performance of multiple functions, the one ormore processors 2704 may be configured to provide a multitaskingenvironment, whereby each of a plurality of functions is implemented asa set of tasks serviced by the one or more processors 2704 as needed ordesired. In one example, the multitasking environment may be implementedusing a timesharing program 2720 that passes control of a processor 2704between different tasks, whereby each task returns control of the one ormore processors 2704 to the timesharing program 2720 upon completion ofany outstanding operations and/or in response to an input such as aninterrupt. When a task has control of the one or more processors 2704,the processing circuit is effectively specialized for the purposesaddressed by the function associated with the controlling task. Thetimesharing program 2720 may include an operating system, a main loopthat transfers control on a round-robin basis, a function that allocatescontrol of the one or more processors 2704 in accordance with aprioritization of the functions, and/or an interrupt driven main loopthat responds to external events by providing control of the one or moreprocessors 2704 to a handling function.

In one implementation, the apparatus 2700 includes or operates as awireless charging device that has a battery charging power sourcecoupled to a charging circuit, a plurality of charging cells and acontroller, which may be included in one or more processors 2704. Theplurality of charging cells may be configured to provide a chargingsurface. At least one coil may be configured to direct anelectromagnetic field through a charge transfer area of each chargingcell. The controller may be configured to cause the charging circuit toprovide a charging current to a resonant circuit when a receiving deviceis placed on the charging surface, detect a change or rate of change involtage or current level associated with the resonant circuit, provide ameasurement slot by terminating the charging current for a period oftime, and determine that the receiving device has been removed from thecharging surface by performing a passive or digital ping procedureduring the measurement slot.

In one example, the controller may be further configured to initiate apassive ping procedure to determine whether the receiving device hasbeen removed from the charging surface. The passive ping procedure maybe performed during the measurement slot and after terminating thecharging current.

In one example, the controller may be further configured to execute adigital ping procedure to determine whether the receiving device hasbeen removed from the charging surface. The digital ping procedure maybe performed during the measurement slot and after terminating thecharging current.

In some implementations, the apparatus 2700 has one or more sensorslocated proximate to an exterior surface of the charging device. Thecontroller may be further configured to receive measurements from theone or more sensors, and measure the voltage or current level associatedwith the resonant circuit when one of the measurements indicatesphysical removal of the receiving device. The sensors may include astrain measuring sensor, an accelerometer, an infrared or ultrasonicsensing element and/or a hall-effect device.

In some implementations, the storage 2706 maintains instructions andinformation where the instructions are configured to cause the one ormore processors 2704 to provide a charging current to a resonant circuitwhen a receiving device is placed on a charging surface of a chargingdevice, determine that a change in voltage or current level associatedwith the resonant circuit indicates a potential removal of the receivingdevice from the charging surface, provide a measurement slot bydecreasing or terminating the charging current for a period of time, anddetermine whether the receiving device has been removed from thecharging surface based on measurement of a characteristic of theresonant circuit during the measurement slot. In one example, the changein the voltage or the current level includes a step change in thevoltage or the current level. A low-pass filter may be used to filtershort-duration or low-magnitude step changes in a signal representingthe voltage or the current level.

In certain implementations, the resonant circuit includes a transmittingcoil in the charging surface. The characteristic of the resonant circuitmay be indicative of coupling between the transmitting coil and areceiving coil in the receiving device. The instructions may beconfigured to cause the one or more processors 2704 to determine thatthe receiving device has been removed from the charging surface when avoltage measured at a terminal of the transmitting coil exceeds athreshold voltage level. In some instances, the threshold voltage levelmay be maintained by a lookup table. In some instances, the thresholdvoltage level may be determined when the transmitting coil iselectromagnetically uncoupled. In some instances, the threshold voltagelevel may be determined when the receiving device is first placed on thecharging surface.

In certain implementations, the instructions may be configured to causethe one or more processors 2704 to determine that the receiving devicehas been removed from the charging surface when a current measured inthe resonant circuit has a magnitude that is less than a thresholdcurrent level. In some instances, the threshold current level ismaintained by a lookup table. In one example, the threshold currentlevel may be determined when no object is electromagnetically coupledwith a coil in the resonant circuit. In another example, the thresholdcurrent level may be determined when the receiving device is firstplaced on the charging surface.

In some implementations, the instructions may be configured to cause theone or more processors 2704 to determine that the receiving device hasbeen removed from the charging surface based on a rate of decay ofenergy stored in the resonant circuit. In some implementations, theinstructions may be configured to cause the one or more processors 2704to use a passive ping procedure to determine whether the receivingdevice has been removed from the charging surface. The passive pingprocedure may be performed during the measurement slot and afterterminating the charging current. In some implementations, theinstructions may be configured to cause the one or more processors 2704to use a digital ping procedure to determine whether the receivingdevice has been removed from the charging surface. The digital pingprocedure may be performed during the measurement slot and afterterminating the charging current.

In certain implementations, the instructions may be configured to causethe one or more processors 2704 to monitor and/or receive measurementsfrom one or more sensors in the charging surface. The instructions maybe configured to cause the one or more processors 2704 to measure thevoltage or current level associated with the resonant circuit after oneof the measurements indicates physical removal of the receiving device.The sensors may include a strain measuring sensor, an accelerometer, aninfrared or ultrasonic sensing element and/or a hall-effect device.

FIG. 28 is a flowchart 2800 illustrating a method for operating acharging surface in accordance with certain aspects of this disclosure.The method may be performed by a controller in the charging surface. Atblock 2802, the controller may provide a charging current to a resonantcircuit when a receiving device is placed on the charging surface. Atblock 2804, the controller may detect a change or rate of change involtage or current level associated with the resonant circuit. At block2806, the controller may provide a measurement slot by terminating thecharging current for a period of time. At block 2808, the controllerdetermine that the receiving device has been removed from the chargingsurface by performing a passive or digital ping procedure during themeasurement slot. If at block 2810, the controller determines that thereceiving device has been removed from the charging surface, then atblock 2812 the controller may terminate the charging current and thecharging cycle associated with the receiving device. If at block 2810,the controller determines that the receiving device has not been removedfrom the charging surface, then the method may continue or resume atblock 2804.

In certain implementations, the controller may initiate a passive pingprocedure to determine whether the receiving device has been removedfrom the charging surface. The passive ping procedure may be performedduring the measurement slot and after terminating the charging current.

In certain implementations, the controller may execute a digital pingprocedure to determine whether the receiving device has been removedfrom the charging surface. The digital ping procedure may be performedduring the measurement slot and after terminating the charging current.

In certain implementations, the controller may receive measurements fromone or more sensors in the charging surface, and may measure the voltageor current level associated with the resonant circuit after one of themeasurements indicates physical removal of the receiving device. Thesensors may include a strain measuring sensor, an accelerometer, aninfrared or ultrasonic sensing element and/or a hall-effect device.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but is to be accorded the full scope consistentwith the language claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.” Unless specifically statedotherwise, the term “some” refers to one or more. All structural andfunctional equivalents to the elements of the various aspects describedthroughout this disclosure that are known or later come to be known tothose of ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed by the claims. Moreover,nothing disclosed herein is intended to be dedicated to the publicregardless of whether such disclosure is explicitly recited in theclaims. No claim element is to be construed under the provisions of 35U.S.C. § 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for” or, in the case of a method claim, theelement is recited using the phrase “step for.”

What is claimed is:
 1. A method for operating a charging device,comprising: transferring power to a receiving device in accordance witha charging configuration, including providing a charging current to aresonant circuit according to the charging configuration, the chargingconfiguration being generated after determining that the receivingdevice has been placed at a first location on a charging surface of thecharging device; detecting a change or rate of change in voltage orcurrent level in the resonant circuit while transferring power to thereceiving device, initiating a measurement slot by terminating thecharging current for a period of time, and performing a passive ping ordigital ping procedure after initiating the measurement slot; resumingpower transfer to the receiving device in accordance with the chargingconfiguration by resuming the charging current to the first resonantcircuit when the passive ping or digital ping procedure indicates thatthe receiving device remains in place on the charging surface; anddiscontinuing power transfer to the receiving device after determiningthat the receiving device has been moved from the first location.
 2. Themethod of claim 1, further comprising: executing the passive pingprocedure to determine whether the receiving device has been removedfrom the charging surface, wherein the measurement slot is terminatedwhen the passive ping procedure is terminated.
 3. The method of claim 1,further comprising: executing the digital ping procedure to determinewhether the receiving device has been removed from the charging surface,wherein the measurement slot is terminated when the digital pingprocedure is terminated.
 4. The method of claim 1, further comprising:receiving measurements from one or more sensors in the charging device;and providing the measurement slot when one of the measurementsindicates physical removal of the receiving device from the chargingsurface.
 5. The method of claim 4, wherein the one or more sensorsinclude a strain measuring sensor.
 6. The method of claim 4, wherein theone or more sensors include an accelerometer.
 7. The method of claim 4,wherein the one or more sensors comprise an infrared or ultrasonicsensing element.
 8. The method of claim 4, wherein the one or moresensors comprise a hall-effect device.
 9. A charging device, comprising:a charging circuit; and a controller configured to: cause the chargingcircuit to transfer power to a receiving device in accordance with acharging configuration, including by providing a charging current to aresonant circuit when the receiving device has been placed at a firstlocation on a charging surface of the charging device; detect a changeor rate of change in voltage or current level associated with theresonant circuit while transferring power to the receiving device,initiate a measurement slot by terminating the charging current for aperiod of time, and perform the passive ping or digital ping procedureafter initiating the measurement slot; resume power transfer to thereceiving device in accordance with the charging configuration byresuming the charging current to the first resonant circuit when thepassive ping or digital ping procedure indicates that the receivingdevice remains in place on the charging surface; and discontinue powertransfer to the receiving device after determining that the receivingdevice has been moved from the first location.
 10. The charging surfaceof claim 9, wherein the controller is further configured to: initiatethe passive ping procedure to determine whether the receiving device hasbeen removed from the charging surface, wherein the measurement slot isterminated when the passive ping procedure is terminated.
 11. Thecharging surface of claim 9, wherein the controller is furtherconfigured to: initiate the digital ping procedure to determine whetherthe receiving device has been removed from the charging surface, whereinthe measurement slot is terminated when the digital ping procedure isterminated.
 12. The charging surface of claim 9, wherein the controlleris further configured to: receive measurements from one or more sensorsin the charging surface; and provide the measurement slot when one ofthe measurements indicates physical removal of the receiving device. 13.The charging surface of claim 12, wherein the one or more sensorsinclude a strain measuring sensor.
 14. The charging surface of claim 12,wherein the one or more sensors include an accelerometer.
 15. Thecharging surface of claim 12, wherein the one or more sensors comprisean infrared or ultrasonic sensing element.
 16. The charging surface ofclaim 12, wherein the one or more sensors comprise a hall-effect device.17. A non-transitory processor-readable storage medium havinginstructions stored thereon which, when executed by at least oneprocessor of a charging circuit in a charging device, cause the chargingcircuit to: cause the charging circuit to transfer power to a receivingdevice in accordance with a charging configuration, including byproviding a charging current to a resonant circuit when the receivingdevice has been placed at a first location on a charging surface of thecharging device; detect a change or rate of change in voltage or currentlevel associated with the resonant circuit while transferring power tothe receiving device, initiate a measurement slot by terminating thecharging current for a period of time, and perform a passive ping ordigital ping procedure after initiating the measurement slot; resumepower transfer to the receiving device in accordance with the chargingconfiguration by resuming the charging current to the first resonantcircuit when the passive ping or digital ping procedure indicates thatthe receiving device remains in place on the charging surface; anddiscontinue power transfer to the receiving device after determiningthat the receiving device has been moved from the first location. 18.The processor-readable storage medium of claim 17, wherein theinstructions cause the charging circuit to: initiate the passive ping ordigital ping procedure to determine whether the receiving device hasbeen removed from the charging surface, wherein the measurement slot isterminated when the passive or digital ping procedure is terminated. 19.The processor-readable storage medium of claim 17, wherein theinstructions cause the charging circuit to: receive measurements fromone or more sensors in the charging surface; and provide the measurementslot when one of the measurements indicates physical removal of thereceiving device.
 20. The processor-readable storage medium of claim 19,wherein the one or more sensors include a strain measuring sensor, anaccelerometer, a hall-effect device, an infrared sensing element or anultrasonic sensing element.